1. Field of Invention
The present invention relates to certain types of apparatus and methods for storing multimedia information to and retrieving multimedia information from a storage medium, such as, for example, a hard disk, magnetic tape, optical disk, and a writable CD.
2. Description of Background Information
Multimedia information is encoded and packetized using an accepted standard. The multimedia information is encoded and packetized for transmission via a transport stream from a source to a destination for decoding and playback. Some of the packets include timing information, which is critical for playback. Typically, the data is transmitted from the source to a destination over a channel having a constant delay. Due to the constant delay, the temporal information carried in the transport stream is preserved.
Media are increasingly utilized and applied for storing and playing video programs in digital form. Such media may include, for example, a hard disk, magnetic tape, optical disk, a writable CD, and any other device capable of storing digitized multimedia information. For example, industries, such as television, movie, and music industries are increasingly employing the use of hard disks. Applications, such as MPEG-2, transform video and audio contents forming a transport stream (TS) of packets of information.
There are many types of packets of information, each identified by a unique identifier. Some of the packets may contain a Program Clock Reference (PCR) field, which carries the timing information that allows the decoder clock to periodically resynchronize components of the video/audio with the assistance of additional time stamps; such as a decode time stamp and a presentation time stamp. The PCR field allows the decoder clock to track the encoder clock so as to not underrun or overrun the decoder's buffer. Timing information is carried only in the PCR packets, which occur at time intervals not exceeding 100 milliseconds.
When the TS is delivered through a constant delay transmission channel directly from the source, such as an MPEG-2 TS encoder 102, to a receptor, such as an MPEG-2 decoder 104, there is no problem in reconstructing the video/audio contents. See FIG. 1. In this arrangement, the decoder extracts the PCR values in the packets and generates the clock using circuitry to form a phase locked loop. The temporal relationship arising from the direct connection allows the receptor to recover the original clock. The intervening packets of information that do not have PCR fields (non-PCR packets) do not interfere with the timing information since the decoder decodes the encoded packets as it receives each packet.
However, when there is no direct connection between the source and the receptor and no constant delay therebetween, it may be impossible to recover the original clock. One such instance is when the video is stored on a hard disk for playback at a later time. FIG. 2 generally depicts the movement of a TS in such a system. For instance, the encoder 102 dictates the arrival of the packets for storage on the hard disk drive 202. The hard disk drive 202 incorporates a timing generator having the clock rate of the encoder used to generate PCR values. The storage apparatus monitors the output of the timing generator and outputs a packet when the PCR value in the packet equals the value in the timing generator. However, for non-PCR packets, there is no timing value for comparison. Therefore, the decoder does not know when to output these non-PCR packets. This may result in an incomplete or fuzzy video or audio playback. Other problems that may arise are loss of color and “jumpy” pictures.
Thus, there is a need for a method and apparatus that allows for the storage of digital packets and for accurate retrieval of the stored information with a certain required timing.